BELFAST, Northern Ireland—A graffiti-covered steel and concrete wall runs between the Falls Road and Shankill Road in west Belfast, separating two communities, one Catholic, one Protestant.

If every one of these “peace walls” in Northern Ireland were arranged in a line, they’d stretch 20 miles, explains tour guide Eamonn McGuckin, who has spent the past 25 years driving tourists back-and-forth between these two communities, trying to explain in an hour how the 30-year civil war known as the Troubles shaped the Northern Ireland he lives in today.

“We call it the Berlin Wall of west Belfast,” McGuckin says, pulling his cab to the side of the road so we can appreciate the sheer scale of the 30-foot structure. Since the 1998 Good Friday Agreement signaled a truce in Northern Ireland’s decadeslong civil war, this peace wall, originally erected in 1963, has only gotten higher, an attempt to stop homemade firebombs from being thrown across.

It’s a physical reminder of the sectarian divide between republicans and loyalists, one that persists even 21 years after the peace agreement.

Now, as the United Kingdom stumbles toward its exit from the European Union, Northern Ireland has found itself at the center of this messy divorce, and there are worries that the hard-won peace could collapse.

The border between Northern Ireland, which is part of the United Kingdom, and the Republic of Ireland, an EU member state, has become a flashpoint for the argument over Brexit within the ruling British Conservative Party, between the United Kingdom and the European Union, and among Catholics and Protestants in the north.

On the Catholic side of the peace wall, a memorial for civilians who died during the Troubles has replaced several Bombay Street homes that were burned down during the riots of 1969.

Katie DeRosa

A stalemate over the withdrawal agreement could cause the United Kingdom to crash out of the European Union without a trade deal, an arrangement likely to harden the border as goods and people would have to be checked as they cross. That could throw a bomb into the Good Friday Agreement, fueling fears it could mobilize paramilitary groups such as the New IRA, which oppose any customs checks between the north and the south.

Like everything else in the north, Brexit has become an issue separated along republican and loyalist lines.

“Unfortunately, most things become a green and orange issue,” says McGuckin, whose sturdy frame has earned him the nickname Big E. He doesn’t disclose his own sectarian leanings until the end of his tour, but the street where he grew up tells you everything you need to know.

The middle child of three sisters, McGuckin, 52, grew up on Falls Road in a Catholic working-class neighborhood in west Belfast. The sound of bombs or gunfire in the middle of the night would send the siblings rolling from their beds onto the floor, and it wasn’t uncommon for British soldiers, who maintained a heavy presence on the road through much of the Troubles, to burst through their home in the midst of a hot pursuit.

McGuckin voted to remain in the European Union, as did more than 55 percent of voters in Northern Ireland.

“I know one thing: If they leave the EU without any deal, it’s going to be horrific, just horrendous for Northern Ireland,” he says as his tour van idles in a Protestant neighborhood off Shankill Road within view of a mural of two Ulster Volunteer Force gunmen pointing assault rifles, the barrels of which follow you like the eyes of the Mona Lisa.

Northern Ireland’s economy, already lagging behind the U.K. and the Republic, is sure to suffer, he said, and the business community, unions, and farmers have warned of the grave consequences of customs checks with Ireland, the U.K.’s largest trading partner. Tourism, McGuckin’s bread and butter, could suffer as well. With no major airport in Northern Ireland, the region depends on tour buses that drive up from Dublin without the hassle of passport checks. Fears of political violence and instability could also put tourists off.

“There’s nothing to be gained from a no-deal Brexit.”

A mural commemorating loyalist paramilitaries in a working-class Protestant community in Belfast.

Katie DeRosa

For Ron McMurray, his entire identity is staked on Brexit.

“I’d rather be poor and British than rich and European,” he says. The 62-year-old is a Protestant and former member of the Ulster Volunteer Force, the loyalist paramilitary group set up as a rival to the IRA. He joined the armed struggle at the age of 16, radicalized after a day Protestants call Bloody Friday—July 21, 1972—when the Provisional IRA let off 20 bombs that killed nine people and injured 130. McMurray spent 12 years in prison after he, in his words, “killed a few people,” but his sentence ended early when the Good Friday Agreement secured the release of political prisoners on both sides.

Tall and wiry with thinning white hair, McMurray now looks more like a pensioner than a paramilitary. His views have softened with age and he now stands alongside former IRA members and former British soldiers to talk to school students about the Troubles.

He’s unwavering in his position that Brexit will allow British people to reclaim their identity, which has been eroded by living in a “Eurostate.”

“I’d rather be poor and British than rich andEuropean.”

We’re sitting in a café just steps from the Europa Hotel, which is still standing despite its moniker as the most bombed hotel in Europe, the scene of 36 bomb attacks during the Troubles.

McMurray’s mother worked as a waitress in the hotel during that time but was never injured.

When the U.K. finally does leave the European Union, the border has to harden, McMurray says, to prevent migrants from using Ireland as a back door to enter the United Kingdom.

The current withdrawal agreement, negotiated by Prime Minister Theresa May, includes the so-called backstop, an insurance policy that prevents a hard border and keeps the U.K. in the EU Customs Union until a permanent arrangement can be worked out for what will become the only land border between Europe and the U.K.

But May has now seen her deal rejected by members of Parliament three times, with hard-line Brexiteers fearing that the backstop would keep the U.K. in the EU Customs Union indefinitely. As the April 12 deadline to leave the EU looms, MPs can’t agree on the type of Brexit they want, and the House of Commons has been deadlocked as leavers and remainers squabble over the future of the U.K.’s relationship with the EU.

That fear of slipping back into a period of armed conflict was heightened last month when the New IRA, a splinter group that opposed the 1998 peace agreement signed by the Provisional IRA, claimed responsibility for four mail bombs sent to buildings in London and Glasgow. It has sparked fears that the group will escalate its attacks if a border is reestablished. A memo from the British government’s top civil servant has also warned that a no-deal Brexit could lead to the return of direct rule over Northern Ireland from London for the first time since 2007.

“I think the dark question is ‘Are we going back to war?’ ” McMurray asks. He shakes his head. “Social conditions have changed enormously. The loyalists won’t go back, unless the dissident IRA starts up their campaign.”

John Brewer, a professor of post-conflict studies at Queen’s University–Belfast, says the threat of violence has been widely exaggerated.

“There’s something of an anti-Irishness in the extent of the British wanting to exaggerate the likelihood of the Irish killing one another again,” he says.

A pro-Brexit mural adorns the side of a Protestant home in west Belfast.

Katie DeRosa

Matt Carthy spent his teen years growing up in Carrickmacross, a town just south of the border. In the 1990s, as his political ambitions were growing, Carthy watched the political violence transition into the peace period and witnessed the heavily militarized border shift into the imperceptible line it is today. Carthy is now a member of the European Parliament for Sinn Féin, the left-wing Irish republican party once known as the political branch of the IRA. Any physical manifestation of the border, Carthy said, will be resisted by local communities.

“Not necessarily violently, but it is certainly going to be resisted to the extent that people won’t accept it ever again,” he says.

Carthy characterizes the Brexit process as “a debacle from start to finish.”

That’s because May’s Brexit priorities were grounded in an inherent contradiction, Carthy said. The prime minister vowed to take Britain out of the Customs Union while at the same time committing that there would be no hard border on the island of Ireland.

“Never throughout the process has the British government put forward a proposition that would actually address that contradiction,” Carthy says.

“I think the dark question is ‘Are we going back to war?’”

Another major hurdle in securing a deal is the fact that May’s minority Conservative government stakes its survival on the support of the Democratic Unionist Party, a Northern Irish right-wing ultraconservative party whose entire political identity can be traced back to its controversial founder, Protestant minister and loyalist politician Ian Paisley. The party vehemently opposes the backstop for fear that it will establish a border down the Irish Sea, thus laying the groundwork for Northern Ireland to join the Irish Republic. The peace agreement requires that Northern Ireland be governed through a power-sharing executive between the DUP and Sinn Féin, but due to a clash between the two parties, there’s been no government at Stormont since January 2017. The two parties are also on opposite sides of the Brexit question.

While Brexiteers went into the referendum with the rallying cry “Take back control”, the eventual outcome, Brewer says, could see the U.K. lose control of Northern Ireland as Brexit has accelerated the talk of a vote on Irish unification. He suspects some middle-class loyalists are moving from a politics of religion to pragmatism, favoring the economic stability of the European Union over staunch support of Brexit based on a British identity.

McMurray and McGuckin might not agree on much, but both men see the possibility that Brexit could open the door for Irish unification. But with the continued uncertainty over what Brexit will look like, McGuckin worries about the political and economic upheaval involved in getting there.

“Are we prepared to have all these casualties before they get it right?” he asks, standing in the shadow of the peace wall.